This application is the U.S. national phase of international application PCT/GB01/02284 filed May 24, 2001 which designated the U.S.
The invention relates to bipolar transistors of the kind which implement exclusion and extraction of minority carriers from the base.
Narrow band gap semiconductors such as indium antimonide (lnSb) are characterised by high electron mobility, high electron saturation velocity and low electron effective mass. These properties are of great interest for high speed device applications, for example high speed transistors. Bipolar transistors fabricated from narrow band gap semiconductors may be operated at high frequencies, and consume less power than conventional transistors because they may be operated with low collector and base voltages.
However, at room temperature, a narrow band-gap semiconductor has a high intrinsic carrier concentration (i.e. electron-hole pairs provided by thermal excitation as opposed to single carrier type produced by ionising a donor or acceptor). Consequently a bipolar transistor fabricated from such semiconductors suffers from high leakage current and high output conductance. This results in low dynamic range and limited voltage gain.
Low dynamic range in a narrow band gap bipolar transistor may be counteracted by use of excluding and extracting contacts at the transistor""s base region, as disclosed in U.S. Pat. No. 5 382 814. This patent discloses an lnSb bipolar transistor with a base region that is intrinsic at room temperature, due to the fact that the background doping level (number of ionised dopant atoms) is lower than the intrinsic carrier concentration. Under operation, minority carriers are extracted from the transistors base at the junctions with the emitter and collector regions. Minority carriers are excluded from entry into the base from the transistors base contact by a wide band gap ln0.85Al0.15Sb region between the base contact and the base. The carrier concentration in the base is therefore reduced well below the intrinsic carrier concentration, reducing leakage current and increasing dynamic range somewhat. However, the dynamic range and high frequency performance of this bipolar transistor are still relatively poor, and the devices are found not to work properly at temperatures approaching ambient temperature, which is necessary for any practical device. The bipolar transistor disclosed in the above patent has a further disadvantage in that it has a high base access resistance because the base region is of low doping, which results in poor power gain.
It is an object of the invention to provide an alternative form of bipolar transistor.
The present invention provides a bipolar transistor having emitter and collector regions arranged to extract minority carriers from the base region, a structure for counteracting entry of minority carriers into the base region via the base contact, the base region and the collector region having a band gap less than 0.5 eV and wherein the base region has a doping level greater than 1017 cmxe2x88x923.
The invention provides the advantages that a transistor of the invention has a greater dynamic range, greater AC voltage and power gain-bandwidth products and a lower base access resistance than prior art narrow-bandgap bipolar transistors.
Transistors of the invention preferably have an emitter region with a larger band gap than that of the base region. This ensures that transistors of the invention provide useful current gain when the doping level of the base is large compared with that of the emitter.
The structure for counteracting entry of minority carriers into the base via the base contact may be an excluding heterostructure, with a barrier to prevent carriers entering the base. Alternatively it may be an implanted region within the base region, providing a xe2x80x9chigh-lowxe2x80x9d doping junction which also inhibits minority carrier entry into the base. This alternative provides for simpler fabrication and allows the use of the wide gap emitter as a passivation layer between the base and emitter regions, if desired. The inclusion of a passivation layer improves performance by reducing recombination in the base. This is known to cause increased leakage and reduced current gain in prior art bipolar transistors, so the reduction of this is beneficial.